give value [] if disallowed_metadata_keys is None

The default value of DiskFile.disallowed_metadata_keys is None. And it
will be iter in following codes even if it didn't get a new value. So
it is necessary to give Diskfile.disallowed_metadata_keys an empty list
in __init__ if the default value None is used.

fixes bug #1206740

Change-Id: I8454eba47b2c691166924356ab33b0235a800951

swift/obj/diskfile.py

@@ -364,7 +364,7 @@ class DiskFile(object):
         self.device_path = join(path, device)
         self.tmpdir = join(path, device, 'tmp')
         self.logger = logger
-        self.disallowed_metadata_keys = disallowed_metadata_keys
+        self.disallowed_metadata_keys = disallowed_metadata_keys or []
         self.metadata = {}
         self.meta_file = None
         self.data_file = None

test/unit/obj/test_diskfile.py

@@ -342,6 +342,12 @@ class TestDiskFile(unittest.TestCase):
                                     FakeLogger(), keep_data_fp=keep_data_fp)
         return df
 
+    def test_disk_file_default_disallowed_metadata(self):
+        keep_data_fp = True
+        df = diskfile.DiskFile(self.testdir, 'sda1', '0', 'a', 'c', 'o',
+                                    FakeLogger(), keep_data_fp=keep_data_fp)
+        self.assertEquals(df.disallowed_metadata_keys, [])
+
     def test_disk_file_app_iter_corners(self):
         df = self._create_test_file('1234567890')
         self.assertEquals(''.join(df.app_iter_range(0, None)), '1234567890')